The applicant, Brian L. Tracy, is a physiologist who is currently supported by an individual National Research Service Award from NIA. Dr. Tracy has developed a record of scholarly work in the area of neuromuscular function and adaptation, with a focus on aging. His career goal is to develop an independent research program with a mission to study: 1) the effects of aging on neuromuscular function; and 2) interventions designed to improve muscle function and functional performance in older adults. To accomplish this goal, Dr. Tracy will propose hypothesis-based research questions to obtain funding to conduct experiments and to support graduate students and postdoctoral fellows. The Mentored Research Scientist Development Award will provide the training necessary to accomplish these goals. Career Development/Training Plan: The training plan comprises three elements: 1) A substantial broadening of research skills; 2) regular participation in journal clubs, seminar series, and scientific meetings; and 3) frequent organized interactions with the Sponsor and established senior investigators and faculty members with expertise in all aspects of the proposed research and career plan. Research Plan: The overall aim of the project is to determine the physiological mechanisms that explain: 1) the reduced steadiness of contractions with knee extensor muscles in older adults; and 2) sex-based differences in the decline in steadiness. The general hypothesis is that increased motor unit size and variability of discharge rate explain differences in steadiness due to age, but that the effect is greater in men. These results will provide information on neuromuscular mechanisms that explain a functionally important aspect of neuromuscular aging. Environment: The environment for the training program was described in the previous review as excellent. The Sponsor, Roger M. Enoka, Ph.D., is an internationally recognized senior investigator and experienced mentor in aging and neural control with a well-funded laboratory. The Sponsor, host department, and interactions with other prominent, funded investigators will provide a rich, supportive environment that will substantially enhance the career development of the candidate.